ANNUAL REPORT
1999
RESEARCH INSTITUTE FOR SOLID STATE
PHYSICS AND OPTICS
Research Institute for Solid State Physics
of the Hungarian Academy of Sciences
Director: Dr. János Kollár
Address: Budapest XII., Konkoly-Thege M. út 29-33, Hungary Letters: H-1525 Budapest, P.O.B. 49
Phone: (36-1-) 395 9153 Fax: (36-1-) 395 9278
E-Mail szfki@power.szfki.kfki.hu URL: http://www.kfki.hu/~szfkihp/
ANNUAL REPORT 1999
Edited by L. Csillag, G. Konczos, B. Selmeci, E. Tóth-Kádár I. Tüttő, Closed on 1st December, 1999
ISSN 1418-4559
Dear Reader,
It is my pleasure to hand over the Annual Report of the research activities of the Research Institute for Solid State Physics and Optics in 1999. This booklet is already the 6th Volume in a series of yearbooks of our Institute.
Our Institute was founded by the Hungarian Academy of Sciences in 1981 as part of the Central Research Institute for Physics. In 1992 we became an independent institute with the name : "Research Institute for Solid State Physics". In 1998 the Crystal Physics Laboratory of the Hungarian Academy of Sciences joined our Institute as a part of the reorganization process of the academic institutes and at the same time the name of the institute has been changed to "Research Institute for Solid State Physics and Optics".
The main profile of the Institute is to do basic research in the fields of theoretical and experimental solid state physics and materials science, including metal physics and liquid crystal research, theoretical and experimental optics, including laser physics and the interaction of light with matter. Our experimental research activity is connected to unique methodologies based on large and medium sized facilities like X-ray diffraction, NMR-, Mössbauer-, and optical spectroscopies and neutron scattering experiments at the KFKI Research Reactor.
Some of our research (R & D) activities are more closely related to applications, first of all in the fields of optical thin films, laser applications, crystal growing technologies and metallurgy.
Our research activity in the field of basic research is financed by the Hungarian Academy of Sciences and the National Research Fund (OTKA) through supporting individual projects. The participation of our research groups in international projects has become more and more significant in supporting our research. Our research groups have gained financial support from the National Committee for Technological Development, too.
In 1999 Hungary has joined the European Union 5th Framework Programme, and thus the international cooperation has become even more important for the scientific work of our research groups. We have living contacts with a great number of research institutions and universities. In more than half of our publications there are foreign co-authors indicating the significant role of these contacts. The different EU, ESF, COST, NATO and other international projects play a rapidly increasing role in our research activity. It is expected that the share of these resources in our budget will increase with the evolution of the integration process of our country. Our Institute has been taking part traditionally in gradual and to a larger extent in postgradual education. Details of this activity are also given in this Annual Report. We have published more than 180 papers in high quality international journals and conference
proceedings. This number of publications (per scientists) is similar to that in the previous years.
May I hope that this booklet gives useful information to the reader. The key figures help you to get a general overview of our Institute as a whole. The Annual Report contains the e-mail addresses of our scientists too, to make it easier to get in contact with them directly. For further information please visit our WEB-page.
Budapest, December 1, 1999
János Kollár
Director
Key figures
Permanent staff of the Institute: 165 employees. Its distribution:
a) by professions:
b) by scientific titles/degrees:
c) by ages:
69%
4%
19% 8%
scientists engineers
technicians/assistants administrators
0 5 10 15 20 25 30
3
11
25
30
14
% under 30 years 30-40 years 40-50 years 50-60 years over 60 years
5 28 47
3 31
51
member of Hungarian Academy of Sciences doctor of science (Dr.
habil.)
Ph.D (candidate of science)
university doctor other
Financial management a) Sources of operation costs:
b) Distribution of expenditures:
62%
9%
11%
5% 13%
MTA (Hungarian Academy of Sciences)
OTKA (Hungarian Scientific Research Fund)
foreign (international) grants
OMFB (National Committee for Technological Development) others (incl. contracts)
14% 40%
16%
3%
15% 12%
wages and salaries
overhead, labour (health service, etc.)
overhead, other (energy, etc.)
consumables
others (incl. travel costs)
investments
A. STRONGLY CORRELATED SYSTEMS
J. Sólyom, G. Fáth, Ö. Legeza, K. Penc, A. Rákos, K. Vladár, F. Woynarovich, A. Zawadowski+
Low dimensional magnetic models. — We have determined the phase diagram of S=1/2 anisotropic spin ladders. It is well established that isotropic antiferromagnetic spin ladders with odd number of legs remain gapless, while for even number of legs a Haldane gap is generated. We have shown that in latter case the two-leg ladder behaves differently from ladders with higher number of legs. In a two-leg ladder the inter-leg coupling is always relevant, generating a Haldane gap. For four-leg (six-leg, etc.) ladders this coupling is in general irrelevant, except for the special isotropic situation.
The gap generation starts only at a finite value of the inter-leg coupling.
We have studied the low energy excitations of CuGeO3 and haveshown that to describe properly, one must include the effects of a transverse antiferromagnetic coupling, which was estimated to be J⊥=0.15 J. Owing to this coupling the frustration in the chains is significantly lower than recent 1D estimates based on purely one-dimensional arguments, and we have found a strong modulation of the nearest neighbour coupling due to the static distortion, which is 5 times higher than that previously deduced from a 1D chain approach. By performing density-matrix renormalization group method (DMRG) calculations for 2 coupled chains we have analysed the effect of the transverse coupling on the ratio of singlet to triplet gaps. The ratio is very sensitive to the parameters and the universality reported in the strict one-dimensional case is lost.
The zero temperature phase diagram of a one-dimensional S=2 Heisenberg ferromagnet with single-ion cubic anisotropy was studied numerically using the DMRG. Evidence was found that although the model does not involve quadrupolar couplings, there is a purely quadrupolar phase for large values of the anisotropy. The phase transition between the magnetic and quadrupolar phases is continuous and it seems to be characterized by Ising critical exponents.
Using the XXX Heisenberg chain as a representative example we have studied the connection between the basic SU(2) symmetry of a model and the symmetry of the excitations. It was found, that the symmetry of the eigenstates can be explained supposing that the excitations obey SU(2) with a modified coproduct, or they obey a q-deformed SU(2) at q=–1. We started to examine the behaviour of the extremly low (O(1/N)) energy excitations of the XXX Heisenberg chain. We have found strong indications that the size of these excitations is of the order of the chain length.
Fermionic models. — Quasi-1D materials based on cuprate compounds, of which Sr2CuO3 is the best example, have become new candidates for ideal model systems which allow the study of basic physical concepts in one-dimension. Information on the electronic structure and the dynamics of the charge carriers is highly desirable, especially against the background of spin-charge separation expected in 1D. Beside the one-particle spectral function obtained by photoemission, the dielectric function is the most basic and important quantity reflecting the electronic structure of a solid. The dielectric response is accessible using electron energy-loss spectroscopy (EELS), offering the possibility to study the momentum dependence of the electronic excitations, i.e. the dynamical dielectric response.
+ Permanent position: Technical University of Budapest
We have reported the first investigations of the dynamical dielectric response of Sr2CuO3 in collaboration with the experimentalists of the Institut für Festkörper- und Werkstofforschung in Dresden, and Department of Superconductivity, The University of Tokyo. We have carried out EELS measurements on single crystalline samples, which provides us with the energy and momentum dependent loss function Im (- 1/ε(q,ω)). While for small momentum transfer we see a broad continuum of interband plasmons above the gap, on the way to the zone boundary a sharp peak develops. We show that the data can be understood within an extended effective one-band Hubbard model and that both the spin-charge separation which occurs in 1D as well as excitonic effects are essential.
It is believed that some aspects of the electronic properties of the strongly correlated transition metal oxides, like manganites, can be revealed by considering the Kondo- lattice Hamiltonian with ferromagnetic exchange between the localized and itinerant electrons (Hund's rule). We have shown that the strong coupling limit of the Kondo lattice with infinite long range hoppings can be solved exactly using the underlying spl(2,1) dynamical supersymmetry. We learned that on this particular lattice (i) an extended Gutzwiller projection becomes exact, (ii) the ferromagnetic ground state is not favored. Finally, extending our result to more general lattices we arrived at an interesting conjecture that large density of states (e.g. Kagomé and pyrochlore lattices have flat bands in their one-particle density of states) is against ferromagnetism in the double exchange model.
Theory of dissipative motion of heavy particles. — We studied the Bloch wave diffusion of a heavy particle in fermionic environment. Path integral formulation and renormalization was derived by integrating on the fermionic degrees of freedom. The heavy particle localizes at strong dissipation, similarly to the case of two state system.
Other topics. — We studied cellular differentiation in a developing organism via a discrete bistable reaction-diffusion model. A system of undifferentiated cells was allowed to receive an inductive signal emanating from its environment. Depending on the form of the nonlinear reaction kinetics, this signal could trigger a series of bifurcations in the system. Differentiation started at the surface where the signal was received and either cells changed type up to a given distance or, under other conditions, the differentiation process propagated throughout the whole domain. When the signal was diminishing, hysteresis was observed.
We analysed the Ginzburg-Landau and Schrödinger equations with a nonuniform magnetic flux density, and showed that they admit a class of internally-orthogonal, - and in special cases - tracking solutions, where the gradient of the logarithm of the order parameter (wave function magnitude) is orthogonal to the gradient of the gauge invariant phase. For the macroscopic GL model virtually all solutions are in the internally-orthogonal class. In two dimensions, we found multi-fluxoid quantum vortex, dot and wall nucleation, and surface tracking solution. For the microscopic Schrödinger model we demonstrated that there exists a broad class of internally orthogonal, and closed form tracking solutions.
Gábor Fáth fath@power.szfki.kfki.hu Örs Legeza olegeza@power.szfki.kfki.hu Karlo Penc penc@power.szfki.kfki.hu Attila Rákos rakos@power.szfki.kfki.hu
Jenő Sólyom solyom@power.szfki.kfki.hu Károly Vladár vladar@power.szfki.kfki.hu Ferenc Woynarovich fw@power.szfki.kfki.hu
Grants
OTKA1 T017128. Theoretical study of dissipative motion of heavy particles (K.
Vladár 1995-1999)
OTKA T022607. Completely integrable quantum chains (F. Woynarovich 1997-2000)
OTKA T030173. Theoretical study of magnetically or electrically low- dimensional models (J. Sólyom, 1999-2002)
MAKA2 JF 555/95-B Unconventional behavior of low-dimensional magnetic and electric systems (J. Sólyom, 1996-1999)
Publications
Articles
A.1. I. Peschel*, M. Kaulke*, Ö. Legeza: Density matrix spectra for integrable models. Annalen der Physik 8, 153-164 (1999)
A.2. Ö. Legeza and J. Sólyom: Response of finite spin-S Heisenberg chains to local perturbations. Phys. Rev. B 59, 3606-3611 (1999)
A.3. E.H.Kim* and J.Sólyom: Opening of the Haldane gap in anisotropic two- and four-leg spin ladders. Phys. Rev. B 60, (1999); cond-mat/9903241
A.4. M. Dudzinski*, G. Fáth, and J. Snajd*: Magnetic and quadrupolar order in a one-dimensional ferromagnet with cubic crystal-field anisotropy. Phys. Rev. B 59, 13764-13774 (1999)
A.5. F. Woynarovich and P. Forgács*: Scaling limit of the one-dimensional attractive Hubbard model: The non-half-filled band case. Nucl. Phys. B 538 [FS] 701-730 (1999)
A.6. P. Millet*, C. Satto*, J. Bonvoisin*, B. Normand*, K. Penc, M. Albrecht* and F.
Mila*: Magnetic properties of the coupled ladder system MgV2O5. Phys. Rev.
B 57, 5005-5008 (1998)
A.7. R. Neudert*, M. Knupfer*, M. S. Golden*, J. Fink*, W. Stephan*, K. Penc, N.
Motoyama*, H. Eisaki* and S. Uchida*: Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3. Phys. Rev.
Lett. 81, 657-660 (1998)
A.8. G. Fáth, Z. Domanski*: Avalanche of bifurcations and hysteresis in a model of cellular differentiation. Phys. Rev. E 60, 4604-4609 (1999)
1 OTKA = Hungarian Scientific Research Fund 2 MAKA = US-Hungarian Joint Fund
A.9. G. Bouzerar*, Ö. Legeza and T. Ziman*: Minimal model to describe the magnetism of CuGeO3. cond-mat/9909375, Phys. Rev. B, accepted for publication
A.10. T. Hauer*, A. Rákos, F. Woynarovich: Scaling limit of the one-dimensional XXZ Heisenberg chain with easy axis anisotropy. cond-mat/9901184, Nuclear Physics B, accepted for publication
A.11. K. Penc and R. Lacaze*: spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models. Europhysics Letters, accepted for publication
A.12. S. B. Haley*, H. J. Fink*, G. Fáth: Internally orthogonal and tracking solutions of the Ginzburg-Landau and Schrödinger equations. Phys. Rev. B, accepted for publication
Conference Proceeding
A.13. E. H. Kim* and J. Sólyom: Evolution of the Haldane gap in anisotropic spin ladders. In: Proc. Physical Phenomena at High Magnetic Field - III, Florida, 1999, World Scientific Publishing Co. Inc., Singapore (1999)
Others
A.14. F. Woynarovich: On the symmetry of excitations in SU(2) Bethe Ansatz systems. cond-mat/9812415
A.15. G. Mihály, I. Kézsmárki*, F. Zámborszky*, M. Miljak*, K. Penc, P. Fazekas, H.
Berger*, L. Forró*: Orbitally Driven Spin Pairing in the 3D Non-Magnetic Mott Insulator BaVS3: Evidence from Single Crystal Studies. cond-mat/9911122
B. COMPLEX SYSTEMS
N. Menyhárd, F. Iglói, R. Juhász+, A. Sütő, P. Szépfalusy+
The principal interest of this group is the theoretical investigation of different aspects of equilibrium and non-equilibrium statistical physics and quantum systems.
Phase transitions and critical behaviour. — We have studied the Brownian motion of a classical particle in one-dimensional inhomogeneous environments where the transition probabilities follow random or aperiodic distributions. We found that the diffusion is generally anomalous: either the diffusion exponent is smaller than one, or the diffusion takes place in logarithmic time-scales. First passage time and persistence properties are also calculated analytically.
We have studied random quantum magnets when the distribution of the couplings and/or fields is either inhomogeneous or correlated. In one-dimensions we have obtained several exact results, which show that the critical and Griffiths-McCoy singularities are generally enhanced by such type of relevant perturbations.
Further - mainly numerical -investigations have been carried out in one-dimensional nonequilibrium systems from the point of view critical properties at directed- percolation-like phase transitions.
Quantum systems. — Semiclassical quantization has been derived for spins of not too small a quantum number S > 5. The 2S+1 eigenvalues of a Hamiltonian, exhibiting resonant tunneling as the magnetic field parallel to the anisotropy axis increases, have been computed. Special attention has been paid to the resonance condition.
Using a new procedure we have determined the excitation energies of a degenerate Bose-gas in a magnetic trap in a wide range of the temperature and the potential anisotropy.
Other topics. — We have shown that the transient chaos has much reacher possibilities comparing to the permanent chaos, since there is a continuum of conditional measures even for a given dynamics. Its effect to the transient diffusion has also been determined.
E-Mail:
Ferenc Iglói igloi@power.szfki.kfki.hu Róbert Juhász juhaszr@sol.cc.u-szeged.hu Nóra Menyhárd menyhard@power.szfki.kfki.hu András Sütő suto@power.szfki.kfki.hu Péter Szépfalusy szepfalusy@ludens.elte.hu
Grants:
OTKA T023642 Phase transitions in quasi-crystals, aperiodic and disordered systems (F. Iglói, 1997-2000)
OTKA T023791 Nonequilibrium phase transitions (N. Menyhárd, 1997-2000) OTKA T030543 Mathematical study of systems of quantum spins and particles
(A. Sütő, 1999-2002)
+ Permanent position: Eötvös Loránd University, Budapest
Publications
Articles
B.1. F. Iglói and E. Carlon*: Boundary and Bulk Phase Transitions in the Two- dimensional Q>4 state Potts model. Phys. Rev. B 59, 3783-3792 (1999) B.2. F. Iglói, L. Turban* and H. Rieger*: Anomalous Diffusion in Aperiodic
Environments. Phys. Rev. E 59, 1465-1474 (1999)
B.3. H. Rieger* and F. Iglói: Average Persistence in Random Walks. Europhys. Lett.
45, 673-679 (1999)
B.4. F. Szalma* and F. Iglói: Two-dimensional Dilute Ising Models: Critical Behavior Near Defect Lines. J. Stat. Phys. 95, 763-770 (1999)
B.5. F. Iglói, R. Juhász and H. Rieger*: Griffiths-McCoy singularities in the Random Transverse-Field Ising Spin Chain. Phys. Rev. B 59, 11308-11314 (1999) B.6. E. Carlon*, F. Iglói, W. Selke* and F. Szalma*: Interfacial Adsorption in Two-
dimensional Potts models. J. Stat. Phys. 96, 531-540 (1999)
B.7. D. Karevski*, R. Juhász, L. Turban* and F. Iglói: Transverse-field Ising spin chain with inhomogeneous disorder. Phys. Rev. B 60, 4195-4204 (1999) B.8. L. Turban*, D. Karevski* and F. Iglói: Extended surface disorder in the quantum
Ising chain. J. Phys. A 32, 3907-3918 (1999)
B.9. H. Rieger* and F. Iglói: Random quantum magnets with long-range correlated disorder: Enhancement of critical and Griffiths-McCoy singularities. Phys.
Rev. Lett. 83, 3741-3744 (1999)
B.10. J.L. van Hemmen* and A. Sütő: Semiclassical quantization and resonance in spin tunnelling. J. Phys. A: Math. Gen. 31, 0029-10043 (1998) and 32, 4321 (1999)
B.11. J. Reidl*, A. Csordás*, R. Graham* and P.Szépfalusy: Finite temperature excitations of Bose gases in anisotropic traps. Phys.Rev. A 59, 3816-3822 (1999)
B.12. N. Menyhárd and G. Ódor*: Nonequilibrium Kinetic Ising Models - Phase Transitions and Universality Classes in 1d. Brasilian Journal of Physics, accepted for publication
B.13. Z. Kaufmann*, A. Németh*, P. Szépfalusy: Critical States of Transient Chaos.
Phys. Rev. E, accepted for publication
C. ELECTRONIC STATES IN SOLIDS
J. Kollár, P. Fazekas, K. Itai, A. Kiss, I. Tüttő, B. Újfalussy, A. Virosztek+ , L. Vitos We have prepared a review article on the energetics of metal surfaces including our earlier studies about the surface, step and kink energies of mono-atomic surfaces.
We have studied the stability of fcc (110) surfaces of some transition and noble metals in detail. Furthermore we have developed an efficient and accurate method to calculate the total energies of bulk metals and surfaces based on the exact muffin-tin orbitals (EMTO) theory developed by O.K. Andersen et al. The EMTO theory can be considered as an improved screened KKR (Korringa-Kohn-Rostoker) method which is able to treat large overlapping potential spheres. In our implementation of the EMTO theory the one electron equations are solved exactly using the Green’s function formalism, and the Poisson’s equation is solved within the spherical cell approximation. To demonstrate the accuracy of the method test calculations have been carried out.
The Airy Gas model of the edge electron gas developed by W. Kohn and A.E.
Mattsson has been used to derive explicit density-dependent kinetic and exchange energy functionals. In this description the energy density becomes accurate where the curvature of the effective potential is negligible, and the total energies are systematically improved over the conventional local density approximation. The moderate impact of our gradient correction to the local density approximation in many cases brings the theoretical results to closer agreement with the experimental data than those obtained from the generalised gradient approximation.
Previously we implemented the so-called Abrikosov boundary condition in the Locally Selfconsistent Multiple Scattering theory. Now we carried the idea one step further, and formulated the theory named Polymorphous Coherent Potential Approximation. The conventional Coherent Potential Approximation is a very successful theory of the electronic structure of random substitutional alloys. However, it was well known, that it has defections treating charge transfers and related quantities.
The new PCPA theory is designed to keep or further enhance all features of the conventional CPA theory and allow a proper treatment of the charge transfers. The success of the new theory has been demonstrated in CuZn and CuPd alloys.
We also studied the Exchange Coupling trough alloys spacers, where we generalised our previous theory for pure metals to include random substitutional binary alloys. We found, that just as in pure metals, the periods of oscillations are determined by the extremal nesting vectors of the Fermi surface. However, there is an additional exponential decay, which is determined by the coherence length at the particular point of the Fermi surface where the nesting vectors are.
The three-dimensional transition metal compound BaVS3 which was studied in co- operation with experimentalists from the Technical University of Budapest, Zagreb, and the EPFL Lausanne, offers a unique collection of exotic features: Mott transition unaccompanied by magnetic order or static distortion; a low-temperature phase transition whose order parameter eludes identification; non-Fermi-liquid behaviour under pressure. Using a 3d1-based model for spin and orbital interactions, we started to develop a coherent picture of the observed phenomena. The intermediate insulating
+ Permanent position: Technical University Budapest
state is described as a classical liquid of valence bonds which arises from thermal averaging over an exponentially large number of effective valence bond solid Hamiltonians.
In the field of high temperature superconductors we finished the interpretation of the Raman scattering data in the high-Tc superconductors, and have concluded that the appearance of the pseudogap in the underdoped samples is independent on the superconducting gap. In spite of the different mechanism, the obtained Raman relaxation rates coincide with the infrared relaxation rates.
Considering the relation between the electron-phonon coupling and transition temperature in YBCO and BSCCO samples we found that the data do not support theories based on phonon mechanism. Within the same model we described successfully the neutron scattering data in cuprates, and investigated vertex corrections to our Nested Fermi Liquid Theory applicable in the normal state..
Optical conductivity in spin density waves (SDW) attracted our attention due to a recent measurement with electric field perpendicular to the conducting chain direction.
We calculated the frequency dependent conductivity in the presence of impurity scattering in SDW and found reasonable agreement with experiment. We also made the first steps towards building up the theory of unconventional SDW by working out the thermodynamics and some of the transport properties of this system.
E-Mail:
Patrik Fazekas pf@power.szfki.kfki.hu Kazumasa Itai itai@power.szfki.kfki.hu János Kollár jk@power.szfki.kfki.hu István Tüttő tutto@power.szfki.kfki.hu Balázs Újfalussy bu@power.szfki.kfki.hu Attila Virosztek viro@power.szfki.kfki.hu Levente Vitos lv@power.szfki.kfki.hu
Grants
OTKA T020030 Interacting electrons in low dimensions (A. Virosztek, 1996-99) OTKA T023390 Ab initio study of the structural stability of solids and surfaces
(J. Kollár, 1997-2000)
OTKA T022609 Ab initio studies of magnetic thin films (B. Újfalussy, 1997- 2000)
OTKA T025505 Competition of ferromagnetism with other collective phenomena in the lattice models for electrons (P. Fazekas, 1998- 2001)
OTKA T019045 Collective excitations in unconventional superconductors (I.
Tüttő, 1997-1999)
TÉT3 D52/96 Raman scattering in unconventional superconductors (I. Tüttő, 1997-1999)
ESF Network Program Electronic structure calculations (J. Kollár, 1998-2002) AKP 98-66 Orbital ordering of electrons in magnetic materials (P. Fazekas,
1999-2000)
3 Intergovernmental Science and Technics Project
Publications
Articles
C.1. A. Virosztek and J. Ruvalds*: Susceptibility scaling and vertex corrections for a Nested Fermi Liquid, Phys. Rev. B 59, 1324-1332 (1999)
C.2. T. P. Devereaux*, A. Virosztek and A. Zawadowski: Neutron scattering and the B1g phonon in the cuprates, Phys. Rev. B 59, 14618-14623 (1999)
C.3. A. Virosztek, B. Dóra* and K. Maki*: Impurity scattering and frequency dependent conductivity in spin density waves, Europhys. Lett. 47, 358-363 (1999)
C.4. V. Vescoli*, L. Degiorgi*, M. Dressel*, A. Schwartz*, W. Henderson*, B.
Alavi*, G. Grüner*, J. Brinckmann* and A. Virosztek: Spin density wave gap in the Bechgaard salts (TMTSF)2X, Phys. Rev. B 60, 8019-8027 (1999)
C.5. M. Opel*, R. Hackl*, T. P. Devereaux*, A. Virosztek*, A. Zawadowski*, A.
Erb*, E. Walker*, H. Berger* and L. Forró*: Physical origin of the buckling in CuO2: electron-phonon coupling and Raman spectra, Phys. Rev. B 60, 9836- 9844 (1999)
C.6. B. Újfalussy, G.M. Stocks*, Xindong Wang*, D.M.C. Nicholson*, W.A.
Shelton*, Yang Wang*, and B.L. Győrffy*: Constrained density functional theory for first principles spin-dynamics, Journal of Applied Physics 85, 4824 (1999)
C.7. L. Vitos, H.L. Skriver*, J. Kollár: The formation energy for steps and kinks on cubic transition metal surfaces, Surface Science 425, 212-223 (1999)
C.8. J. Kollár, L. Vitos, B. Johansson*, H.L. Skriver*: Metal surfaces: surface, step and kink formation energies, Physica Status Solidi (6) 217, No.1. (1999) C.9. M. Opel*, R. Nemetschek*, C. Hoffman*, R. Philipp*, F.P. Müller*, R. Hackl*,
I. Tüttő, A. Erb*, B. Revaz*, E.Walker*, H. Berger*, L. Forró*: Carrier relaxation, pseudogap and superconducting gap in high-T cuprates: A Raman scattering study. cond-mat/9908208, Phys. Rev. B, accepted for publication C.10. M. Opel*, R. Nemetschek*, F. Venturini*, R. Hackl*, I, Tüttő, H. Berger, L.
Forró*, A. Erb*, B. Revaz* and E. Walker*: A light scattering study of dynamical carrier properties in cuprate systems, Ferroelectrics, accepted for publication.
C.11. L. Vitos, H.L. Skriver*, B. Johansson*, J.Kollár: Application of the exact muffin-tin orbitals theory: the spherical cell approximation, Comp. Mat.
Science, accepted for publication
C.12. L. Vitos, B. Johansson*, H.L. Skriver*, J. Kollár: Stability of fcc(110) transition and noble metal surfaces, Comp. Mat. Science, accepted for publication
C.13. L. Vitos, B. Johansson*, J. Kollár, H.L. Skriver*: The local kinetic energy density of the Airy Gas. Phys. Rev. A, accepted for publication
C.14. B. Dóra*, A. Virosztek and K. Maki*: The optical conductivity in spin density waves. J. Physique, accepted for publication
C.15. B. Dóra* and A. Virosztek: Unconventional spin density waves in quasi one- dimensional systems J. Physique, accepted for publication
C.16. A. Virosztek, B. Dóra* and K. Maki*: Microwave conductivity in spin density waves. Ferroelectrics, accepted for publication
Book, bookchapter
C.17. P. Fazekas: Lecture Notes on Electron Correlation and Magnetism, Series in Modern Condensed Matter Physics, Vol. 5, pp. 1-777, World Scientific, Singapore, 1999.
C.18. J. Kollár, L. Vitos, H.L. Skriver*: From the ASA towards the full potential Lecture Notes in Physics, Springer Series, pp.1-28, accepted for publication Other
C19. G. Mihály*, I. Kézsmárki*, F. Zámborszky*, M. Miljak*, K. Penc, P. Fazekas, H. Berger* and L. Forró*: Orbitally Driven Spin Pairing in the 3D Non- Magnetic Mott Insulator BaVS3: Evidence from Single Crystal Studies, cond- mat/9911122.
D. NON-EQUILIBRIUM ALLOYS
I. Vincze, J. Balogh, L. Bujdosó, D. Kaptás, T. Kemény, L.F. Kiss
Structure and magnetic properties of nanocrystalline soft magnets. — Nanocrystalline Fe92-xZr7BxCu1 (2 ≤ x ≤ 23) alloys were prepared by a heat treatment up to the first crystalline stage of the amorphous ribbons. The nanocrystalline state and the grain sizes of the nanosize ferromagnetic bcc precipitates embedded in the ferromagnetic residual amorphous matrix were determined by X-ray diffraction. 57Fe Mössbauer spectroscopy was used to estimate the relative amount of Fe in the bcc phase. The combination of the bcc grain size and the quantity of the bcc phase allows the determination of the characteristic thickness of the residual amorphous phase which was found to be about 4 nm, independent of composition. The iron content of the residual amorphous phase was estimated to be about 67 at.% from the amount of the precipitated bcc phase determined from the Mössbauer measurements.
The Curie temperature of the residual amorphous phase, aTc is determined in this two phase-system by Mössbauer spectroscopy via the extrapolation of the temperature dependence of its hyperfine field. An unusual composition dependence was found with a minimum around x=12, the increase of the Curie temperature has reached about 200 K both for decreasing and increasing Zr content (Fig. 1). This dependence is to be contrasted with the continuous decrease with increasing Zr content, expected and observed for the bulk-size amorphous alloys in good agreement with that of aTc at high B concentrations. Since the characteristic thickness of the residual amorphous phase is composition independent, the unusual Zr-content dependence of the aTc
increase cannot be explained by the assumed penetration of the exchange interaction between the bcc granules. The anomalous increase of aTc is attributed to magnetovolume effects.
Fig. 1. Curie temperature of the residual amorphous tissue of the Fe92-xZr7BxCu1
nanocrystals (dots) together with that of melt- quenched Fe2(B1-yZry) amorphous alloys (empty circles, x=7(1-y)/y).
In the B-rich region, between x=12 and 23 the width of the lines of the Mössbauer spectra of the bcc phase taken at high temperatures (i.e. above the Curie temperature of the residual amorphous phase) displayed significant reversible increase with increasing temperature: an obvious indication of magnetic relaxation (Fig.2). From the composition dependent starting temperature (400-600 K) to about 800 K the temperature dependence of the line broadening could be well approximated by an Arrhenius type expression which yields the magnetic anisotropy energy, Ean of the nanosize ferromagnetic grains. In the simplest superparamagnetic approximation the anisotropy energy is Ean=KV (K is the uniaxial magnetic anisotropy constant, V is the volume of the relaxing particle). The superparamagnetic behaviour was verified also by magnetic measurements and the characteristic magnetic size of the relaxing particles was determined from the usual Langevin-type H/T plots.
0 5 10 15 20 25
0 200 400 600
x Tc [K]
Fig. 2. Temperature dependence of the Mössbauer spectra of the low B (x=4) and the high B (x=15) content Fe92-xZr7BxCu1 nanocrystals, respec- tively. The separated components of the amorphous (dotted line), the interlayer (chain line) and the bcc (dashed line) are also shown.
Grain boundary structure of iron. — Mössbauer spectra of different nanocrystalline samples are often interpreted according to the picture that they contain an extended grain-boundary region which is very different from that of the usual polycrystalline grain boundaries and has a significantly reduced density. However, increasing number of evidences are collected recently by high resolution transmission electron microscopy and X-ray fine structure measurements which confirm that significant structural disorder at the grain boundary extends no further than the planes immediately adjacent to the boundary plane. Therefore a critical review of the available Mössbauer data and a comparison of results obtained on samples prepared by different methods are important. Samples prepared by ball-milling of iron powder, partial crystallization of Fe-Zr-B-Cu amorphous ribbons and vacuum evaporation of Fe-B and Fe-Ag polycrystalline multilayers were examined and the Mössbauer spectra carefully analyzed as regarding possible impurities and chemical mixing at interfaces.
These results set a limit to the difference between the hyperfine field of Fe atoms in the grain boundary region and in the bulk which is in the order of the linewidth and show that Mössbauer spectra of different nanocrystalline iron samples can be understood without supposing an extended region of the grain boundary with very distorted structure.
E-mail:
Imre Vincze vincze@power.szfki.kfki.hu Sára Judit Balogh baloghj@power.szfki.kfki.hu László Bujdosó bujdi@power.szfki.kfki.hu Dénes Kaptás kaptas@power.szfki.kfki.hu Tamás Kemény kemeny@power.szfki.kfki.hu László Ferenc Kiss kissl@power.szfki.kfki.hu
Grants
OTKA T 020624 The photon- and electron-spectroscopic study of the interfaces of layer structures (J. Balogh, 1996-1999)
OTKA T 022413 The atomic and magnetic structure of nanosystems and interfaces (I. Vincze, 1997-1999)
T =5 00K
T =6 00K
T =7 00K
-8 -4 0 4
ve lo city [m m /s]
a ,
T =7 50K
-4 0 4 8
b ,
OTKA F020092 The investigation of near-surface layers by spectroscopic and diffraction methods (D. Kaptás, 1996-1999)
OTKA T 030753 Magnetic systems with nanoscale inhomogeneities (I. Vincze, 1999-2002)
AKP 98-25 2,2 The relation of magnetic properties and the grain structure in nanocrystals (T. Kemény, 1999-2000)
Publications
Articles
D.1. D. Kaptás, T. Kemény, J. Balogh, L. Bujdosó, L.F. Kiss, T. Pusztai and I. Vincze: Magnetic properties of melt-quenched amorphous Fe2(B1-yZry) (0≤y≤0.55), J. Phys.: Condens. Matter 11, L65-70 (1999).
D.2. T. Kemény, D. Kaptás, J. Balogh, L.F. Kiss, T. Pusztai and I. Vincze:
Microscopic study of the magnetic coupling between nanoscale phases in a nanocrystalline soft magnet, J.Phys.: Condens. Matter 11, 2841-2847 (1999) (see also E.13.)
D.3. J. Balogh, T. Kemény, I. Vincze, S. Szabó*, D. Beke* and J. Tóth*: Comment on the “Grain boundary structure and magnetic behavior in nanocrystalline ball-milled iron”, Phys. Rev. B59, 14786-14787 (1999)
D.4. D. Kaptás, T. Kemény, J. Balogh, L. Bujdosó, L.F. Kiss, T. Pusztai and I.
Vincze: Anomalous magnetic properties of the nano-size residual amorphous phase in nanocrystals, J. Phys. Condens. Matter 11, L179-185 (1999)
D.5. S.C. Yu*, Y.Y. Song*, L.F. Kiss and I. Vincze: Study on the magnetic behavior of nanogranular Cu80Fe10Co10 solid solution, J. Magn. Magn. Mat. 203, 316- 318 (1999)
D.6. J. Balogh, D. Kaptás, T. Kemény, I. Vincze and G. Radnóczi*: Comment on
"Magnetically Ordered fcc Structure at the Relaxed Grain Boundaries of Pure Nanocrystalline Fe", Phys. Rev. Letters 82, 4150 (1999)
D.7. J. Tóth, L.F. Kiss, E. Tóth-Kádár, A. Dinia*, V. Pierron-Bohnes* and I.
Bakonyi: Giant magnetoresistance and magnetic properties of electrodeposited Ni81Cu19/Cu multilayers, J. Magn. Magn. Mater. 198-199, 243-5 (1999) (see also I.8.)
D.8. A. Lovas, L.F. Kiss and I. Balogh: Low-temperature saturation magnetization and amorphous Curie point shift during the early stage of amorphous- nanocrystalline transformation, J. Magn. Magn. Mat., accepted for publication (see also J.14.)
D.9. J. Balogh, L. Bujdosó, D. Kaptás, T. Kemény, I. Vincze, S. Szabó* and D.L.
Beke*: Mössbauer Study of the Interface of Iron Nanocrystallites, Phys. Rev. B, accepted for publication
D.10. J. Balogh, L. Bujdosó, D. Kaptás, T. Kemény, I. Vincze, S. Szabó* and D.L.
Beke*: Hyperfine field at grain boundary atoms in iron nanostructures, Hyperfine Int., accepted for publication
D.11. T. Kemény, D. Kaptás, L.F. Kiss, T. Pusztai, J. Balogh and I. Vincze: The composition dependence of the structural and magnetic properties in Fe92-xZr7BxCu1 nanocrystals. J. Magn. Magn. Mat., accepted for publication Conference Proceedings
D.12. K. Lázár*, L.F. Kiss, S. Pronier*, G. Onyestyák*and H.K. Beyer*: Iron nanoparticles in X and Y zeolites prepared by reduction with NaN3, In: Proc.
NATO Advanced Research Workshop on Mössbauer Spectroscopy in Materials Science, Senec, Slovakia, 6-11 September 1998, Mössbauer spectroscopy in materials science, NATO Science Series: 3. High Technology, Vol. 66, eds. M.
Miglierini and D. Petridis (Kluwer Academic Publishers, London, 1999) pp.
291-8
D.13. A.R. Wildes*, N. Cowlam*, S. Al-Heniti*, L.F. Kiss and T. Kemény: A polarised neutron scattering study of two samples of Fe90Zr10. In: Proc of the European Conference of Neutron Scattering, ECNS'99 (Budapest, 1-4 September 1999), accepted for publication
E. X-RAY DIFFRACTION
G. Faigel, G. Bortel, L. Gránásy, Z. Jurek, K. Kamarás, G. Oszlányi, S. Pekker, T.
Pusztai, M. Tegze
Fullerenes and their compounds. - The fullerenes are closed shell molecules containing only carbon atoms. The most abundant among them is the C60 molecule.
Even in the simplest form (ie. solids of pure C60) C60 is not fully understood.
Illumination of the fcc pristine C60 by intensive light results in a phototransformation.
This was the first case when intermolecular linkage of the fullerene molecules was proposed. Now it is well established that the bonding of C60 molecules in this material occurs through [2+2] cycloaddition. However, the structure of phototransformed C60
is not yet known. This is caused by two facts: first, photopolimerizations were done on thin films, therefore only very small amount of material is available for structural studies. Second, preliminary studies predict an inherent disorder in the bonding pattern of phototransformed C60, which makes structural description difficult. To overcome the above problems we worked out a technique, which produces phototransformed C60 in bulk quantities. This allowed more precise x-ray powder diffraction studies and also differential scanning calorimetry measurements, which was not possible before. Based on the results of these measurements we suggested a structural model in which small closed oligomers (trimers and tetramers) build up the material.
Fullerenes can form a large variety of compounds with elements or other molecules.
In the group of AxC60 compounds (A=K, Rb, Cs x=1,3,4,6) there are materials with very interesting properties. Among them many superconducting materials (A3C60) with remarkably high critical temperature were found. Although the AExC60 type materials (AE=alkaline-earth metal) also show superconductivity, they are much less investigated. This is partly due to their more complicated structure and partly to the high melting point of the alkaline-earth metals, which makes the direct solid state chemical reaction problematic. We tried a simpler synthetic route to these compounds, the synthesis in liquid ammonia. As a test case we have chosen the Ca4C60 composition, which is in the middle of a range where solid solution behavior with face centered cubic structure were reported. We found that a new Ca4(NH3)C60
compound is formed by the inclusion of a single NH3 molecule in the octahedral site of the fcc lattice. We expect that a series of compounds of this family can be synthetised.
X-ray holography with atomic resolution - In holography, the scattered radiation is mixed with a reference wave and the resulting interference pattern is recorded. The hologram contains both the intensity and the phase information and the 3 dimensional image of the object can be reconstructed. The most important limitation of this imaging technique is the spatial resolution, which is given by the wavelength and/or by the source size. Using x-rays for hologram forming and the atoms of the sample as sources or detectors atomic resolution can be achieved. We were the first to demonstrate experimentally the feasibility of x-ray holography with atomic resolution in 1996. To make this tecnique usable in practice further developments are necessary. First of all the data acquisition time has to be decreased. Therefore we implemented this method at synchroron sources. This resulted in measuring times in the range of 30 minutes. The second problem is the very anisotropic resolution in earlier measurements. We solved this by the extension of the hologram to the full
solid angle using the measured symmetry information of the Kossel and standing wave line patterns. An example is shown in Fig.1.
Fig.1. Standing wave pattern and holograms of a CoO sample(left panel).
Reconstructed image of the Co atoms in 3D (right panel).
Theory of phase transformations. We investigated crystal nucleation in various systems using cluster dynamics and continuum models, and crystal growth in the framework of the phase-field theory.
To clarify the mechanism of cross-interfacial molecular transport and the role of subcritical cluster population in determining the kinetics of crystal nucleation, cluster dynamics calculations based on viscosity-governed rate coefficients were confronted with experiments on crystal nucleation in stoichiometric oxide glasses. Systematic deviations have been observed in the temperature dependencies of the measured and predicted induction times that amount to orders of magnitude, and cannot be removed by considering the size-dependence of the interfacial free energy, the depletion of the monomers, or by enforcing the proper value of the free energy of monomers. Rather, it appears that while crystal nucleation and viscosity are both diffusion related processes, they are governed by different diffusion modes.
We calculated the free energy of ice nuclei and the nucleation rate in undercooled water in the framework of a single-order-parameter Cahn-Hilliard theory. The coefficients of the quartic free energy-order parameter relationship and of the square- gradient term were chosen so as to reproduce the measured Gibbs free energy difference, the ice/water interfacial free energy, and the interface thickness predicted by molecular dynamics simulations. Without adjustable parameters, the model reproduces fairly the experimental nucleation rates down to the deepest undercooling so far attained (~ −73 °C). The temperature and size-dependencies of the interfacial properties (free energy, Tolman-length, etc.) have been determined.
In cooperation with team H, we investigated the dynamic response of dendritic growth to time periodic forcing by modulated pressure and Joule heating in the framework of the phase-field theory. It has been shown that the dendritic morphology can be regularized in the typical frequency range of thermal dendritic side-branching. The experiments on liquid crystals support the predictions.
Gábor Bortel gb@power.szfki.kfki.hu Gyula Faigel gf@power.szfki.kfki.hu László Gránásy grana@power.szfki.kfki.hu Zoltán Jurek jurek@power.szfki.kfki.hu Katalin Kamarás kamaras@power.szfki.kfki.hu Gábor Oszlányi go@power.szfki.kfki.hu Sándor Pekker pekker@power.szfki.kfki.hu Miklós Tegze mt@power.szfki.kfki.hu Tamás Pusztai pusztai@power.szfki.kfki.hu
Grants
OTKA F020027 X-ray studies of anisotropic and modulated structures (G.
Oszlányi 1997-2000)
OTKA T022041 X-ray holography (M. Tegze, 1997-2000)
OTKA T025139 Theoretical investigation of the dynamics of nucleation and growth processes (L. Gránásy 1998-2001)
OTKA T029931 Structural studies of polymer fullerides (G. Faigel 1999-2002) OTKA T019139 The study of polymer fullerides and other crystalline C60
compounds (S. Pekker1997-1999)
OMFB4 02264/98 New frontiers in structural biology (G. Faigel 1999-2000) EU CT980377 New frontiers in structural biology (G. Faigel 1999-2000)
Publications
Articles
E.1. M.Tegze, G. Faigel, S. Marchesini*, M. Belakhovsky*, A.I. Chumakov*: Three dimensional imaging of atoms with isotropic 0.5 Å resolution, Phys. Rev.Lett.
82, 4847-4851 (1999).
E.2. L. Gránásy: Nucleation: free energy of small clusters. (Invited for special issue.) Int. J. Non-Equilibrium Processing, 11, 113-139 (1998).
E.3. L. Gránásy: Semiempirical van der Waals/Cahn-Hilliard theory: The size dependence of the Tolman-length. J. Chem. Phys. 109, 9660-9663 (1998).
E.4. L. Gránásy, P.F. James*: Non-classical theory of crystal nucleation:
Application to oxide glasses: Review. J. Non-Cryst. Solids. 253, 210-230 (1999).
E.5. L. Gránásy, P.F. James*: Transient nucleation in oxide glasses: The effect of interface dynamics and subcritical cluster population. J. Chem. Phys. 111, 737- 749 (1999).
E.6. L. Gránásy: Cahn-Hilliard-type density functional calculations for homogeneous ice nucleation in undercooled water. (Invited contribution to the L.S. Bartell issue.) J. Mol. Struct. 485-486, 523-536 (1999).
4 National Commitee for Technological Development
E.7. L. Börzsönyi, T. Tóth-Katona, Á. Buka, L. Gránásy: Dendrites regularized by spatially homogeneous time-periodic forcing. Phys. Rev. Lett. 83, 2853-2856 (1999). (See also H.3.)
E.8. T. Pusztai, G. Oszlányi, G. Faigel, K. Kamarás, L. Gránásy, S. Pekker: Bulk structure of phototransformed C60 , Solid State Commun. 111, 595-599 (1999) E.9. G. Faigel and M. Tegze: X-ray holography. Report on Progress in Physics, 62,
355-392 (1999)
E.10. M.C. Miller*, G. Oszlányi, K.S. Ackroyd*, C. Marshall*, S.P. Collins*, D. Laundy*, R.J. Cernik*: PINCER: a portable data-acquisition program based on a command language interpreter, J. Appl. Cryst., 31, 972-973 (1998) E.11. S.P. Collins*, B.M. Murphy*, C.C. Tang*, G. Oszlányi: SRS Station 16.3:
description and utilization of a high-resolution diffraction facility. J. Phys. D.
Appl. Phys. 32, A81-A83, (1999)
E.12. V.C. Long*, J.L. Musfeldt*, K. Kamarás, A. Schilder*, W. Schütz*: Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60
tetraphenylphosphoniumiodide. Phys. Rev. B 58, 14338-14348 (1998)
E.13. T. Kemény, D. Kaptás, J. Balogh, L.F. Kiss.T.Pusztai, I. Vincze: Microscopic study of the magnetic coupling in nanocrystalline soft magnet. J. Condens.
Matter 11, 2841-2847, (1999) (See also D.2.)
E.14. V.C. Long*, J.L. Musfeldt*, K. Kamarás, A. Schilder*, W. Schütz*: Far-infrared study of C60-tetraphenylphosphoniumiodide. Synthetic Metals 103, 2435-2436 (1999).
E.15 G. Faigel: Holography with Resonant Quanta. Hyperfine Interaction, accepted for publication.
E.16. L. Gránásy, D.W. Oxtoby*: Cahn-Hilliard theory with triple-parabolic free energy: I. Nucleation and growth of a stable crystalline phase. J. Chem. Phys., accepted for publication.
E.17. L. Gránásy, D.W. Oxtoby*: Cahn-Hilliard theory with triple-parabolic free energy: II. Nucleation and growth in the presence of a metastable crystalline phase. J. Chem. Phys., accepted for publication.
Conference proceedings
E.18. T. Pusztai, G. Oszlányi, G. Faigel, K. Kamarás, L. Gránásy, S. Pekker:
Structure of Phototransformed C60 revisited. In: Proc. of XIII. International Winter School on the Electronic Properties of Novel Materials, 1999 Kirchberg Austria, Eds. H. Kuzmany, J. Fink, M. Mehring and S. Roth, pp. 20-23
E.19. G. Oszlányi, S. Pekker, G. Faigel, L. Forró*: Structure of Ca4(NH3)C60. In:
Proc. of XIII. International Winter School on the Electronic Properties of Novel Materials, 1999 Kirchberg Austria, Eds. H. Kuzmany, J. Fink, M.
Mehring and S. Roth, pp. 78-81.
E.20. S Pekker, J.-P. Salvetat*, E. Jakab*, J.-M. Bonnard*, L. Forró*: Chemical Functionalization of Carbon Nanotubes. In: Proc. of XIII. International Winter School on the Electronic Properties of Novel Materials, 1999 Kirchberg Austria, Eds. H. Kuzmany, J. Fink, M. Mehring and S. Roth, 1999, pp. 474- 477.
E.21. D.M. Herlach*, W. Bender*, L. Gránásy, A. Garcia-Escorial*, A.L. Greer*, M.
Kolb*, W. Kurz*, W. Löser*, A. Ludwig*, P.R. Sahm*, B. Vinet*: Undercooled melts: science, technology and application. In: Proc. 2nd European Symp. on Utilization of the International Space Station ESTEC, Noordwijk, The Netherlands, 16 - 18 November 1998 (ESA SP-433, February 1999) pp. 315- 322.
Others (technical digests, papers in Hungarian)
E.22 L. Gránásy, Nukleáció : A vírus kristályoktól a világegyetem szerkezetéig, (Nucleation: from virus crystals to the structure of the Universe, in Hungarian).
In: Lectures of the General Meeting 1999, Hungarian Academy of Sciences, accepted for publication
E.23 G. Faigel: A röntgensugárzás hatása hétköznapjainkra (X-rays in our everyday life), Debreceni Szemle (1999) (in Hungarian), accepted for publication
F. ELECTRON CRYSTALS
G. Kriza, I. Pethes, P. Matus, G. Mihály+, B. Sas
Vortex motion in type-II superconductors. — The defining properties of superconductors are zero resistance and Meissner effect (i.e., the expulsion of magnetic field). Both may fail in Type-II superconductors where the magnetic field penetrates into the bulk in form of vortices and the motion of vortices may lead to dissipative transport. The understanding of the properties of the vortex system, therefore, is a fundamental problem in the physics of these superconductors. We have investigated the dissipative transport in the vortex glass state of the High-Tc
superconductor Bi2Sr2CaCu2O8 using short current pulses. Field cooled preparation shows a higher critical current for dissipation than zero field cooled one but is metastable and convertible to the zero field cooled response by a small field excursion.
The metastability appears on the low temperature side of a peak found in the temperature dependence of the critical current. We suggest that the onset of metastability signals a new thermodynamic ground state.
Nuclear magnetic relaxation in charge-density wave systems. — One of the specific predictions of the Bardeen-Cooper-Schrieffer theory of superconductivity was the existence of coherent electron-electron pairs manifested in so-called coherence effects such as the Hebel–Slichter anomaly in the nuclear spin-lattice relaxation rate.
Such coherence effects have never been demonstrated in any other symmetry breaking ground state of the electronic system. We have found evidence of a quantum coherence peak in the temperature dependence of the 87Rb NMR spin-lattice relaxation rate in the charge density wave system Rb0.3MoO3. The full exploitation of these results is under way.
E-mail:
Ildikó Pethes pethes@power.szfki.kfki.hu György Kriza kriza@power.szfki.kfki.hu Péter Matus matus@wigner.phy.bme.hu György Mihály mihaly@wigner.phy.bme.hu Bernadette Sas bsas@power.szfki.kfki.hu
Grants
OTKA TO23786 NMR investigation of collective electronic states in organic conductors (G. Kriza, 1997-2000)
OTKA T029877: Vortex motion in type-II superconductors (G. Kriza, 1999-2001) BALATON F-24/97 Periodic system in random field (B. Sas, 1998-1999)
French-Hungarian bilateral project TÉT F-24/97: Periodic systems in random potential (B. Sas, 1997-1999)
+ Permanent position: Institute of Physics, Technical University, Budapest
Publications
Articles
F.1 G. Kriza, G. Szeghi*, I. Kézsmárki*, and G. Mihály: Field scaling and
exponential temperature dependence of the magntoresistance in (TMTSF)2PF6. Phys. Rev. B 60, R8434 (1999).
F.2 P. Matus, P. Bánki, and G. Kriza: 87Rb NMR spin-lattice relaxation in the charge-density wave phase of Rb0.3MoO3. J. Physique IV 9, Pr10-235 (1999) F.3 B. Sas, F. Portier,* I. Pethes, K. Vad,* B. Keszei,* L. F. Kiss, I. Puha,* S.
Mészáros,* and F. I. B. Williams*: Metastability line in the vortex phase of BSCCO. J. Physique IV 9, Pr10-45 (1999)
G. LIQUID CRYSTAL RESEARCH
L. Bata, N. Éber, K. Fodor-Csorba, A. Jákli, E. Szabó, A. Vajda
Synthesis and properties of new liquid crystals. — A new synthetic pathway was developed for the preparation of isotope labeled (S)-(-)-4-[(2- methylbutyloxycarbonyl)phenyl-4-n-heptylbiphenyl carboxylate-d8 (MBHB-d8). In magnetic field a µSmC* subphase formed in the SmC* phase, where the orientational order dropped dramatically and increased again slowly with lowering the temperature.
The proton analogue, MBHB, was prepared for electric polarization measurements and showed the same anomaly in the same temperature range. The unwounding of the helical structure and the change in the sign of the polarisation in the SmC* could explain the observed phenomenon.
MBHB-D8
* CH3
COOCH2CHCH2CH3 C7H15 COO
D D
D D
D D
D D
Dielectric spectroscopy measurements were carried out on the formerly synthesised deuterium labeled chiral ferroelectric monomeric and side chain polymeric liquid crystals.
Achiral bent shaped cinnamic acid derivatives were prepared. The substituent effect on the mesophase behavior was investigated with the preparation of chlorine and methyl group containing derivatives. These groups were connected to the central ring, lowering the symmetry of the molecule and enhancing the mesophase formation ability.
Liquid crystals and their miscibility studies. — Four new chiral homologous series (DMn/m) S-(-)-4-(2-n-alkoxy-propionyloxy)biphenyl-4’-n-alkoxy-3,5- dimethylbenzoate having O-substituted S-(-)-lactic acid as terminal group were investigated by optical method. These LCs exhibited monotropic chiral smectic C and chiral nematic mesophases, and two of them blue phase as well. Binary mixtures of these compounds showed enantiotropic ferroelectric mesophases.
Structurally different chiral and non chiral binary and multicomponent mixtures having broad temperature range of enantiotropic ferroelectric mesophase were prepared for physical-electrooptical measurements. In order to understand more deeply the packing of the molecules in the mesophases of the mixtures, X-ray measurements were performed.
Study of ferroelectric liquid crystals. — Electro-optical and textural observations were carried out on a number of materials containing banana-shaped molecules. The voltage dependence of the net polarization indicated that the antiferroelectric ground state is weak, especially in the chiral domains. It was also observed that under long term application of strong fields a racemic state could be transformed to chiral. After field-removal the chiral state is metastable and the stable racemic structure recovers only slowly by a nucleation process that depends both on material parameters,
temperature and surface effects. The figure summarizes the morphological transitions observed in the B2 phase of banana-shaped molecules.
Morphological transitions of the B2 phase of banana-shaped molecules. Dashed arrows indicate relaxation processes. E means electric field treatment, M stands
for the mechanical shear and T means the temperature-induced process.
Novel structures of liquid crystals. — Formation of coils is common in nature when achiral symmetry breaking occurs. Spectacular examples of single, double and triple coils were observed in smectic liquid crystal phases of achiral banana-shaped molecules. The number of observed left - and right-handed domains is equal reflecting the achiral nature of the constituent molecules. These studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.
A polymorphism was observed in 4’-n-octyloxy 4-cyanobiphenyl (8OCB) which brings it into the line with the other homologues. Keeping the material between 35oC and 38oC after it was cooled from above the melting point of the stable crystal form (55oC) regular platelets grow from usually one point and fill the whole sample. The texture is stable in room temperature for weeks. Upon heating it shows a melting point at 50oC, i.e. 5 degrees below the stable crystalline melting point. It is interesting that all platelets are non-symmetric and have the same handedness.
E-Mail:
Lajos Bata bata@power.szfki.kfki.hu Nándor Éber eber@power.szfki.kfki.hu
Katalin Fodor-Csorba fodor@power.szfki.kfki.hu Antal Jákli jakli@power.szfki.kfki.hu
Anikó Vajda vajda@power.szfki.kfki.hu Edit Szabó szedit@power.szfki.kfki.hu
Grants:
OTKA T-020905 Deuterium labeling of liquid crystals (K. Fodor-Csorba, 1996- 1999)
OTKA T-030401 Synthesis of aromatic and heteroaromatic liquid crystals and study of their physico-chemical properties (K.Fodor-Csorba, 1999-2002)
